Reactive spark plasma-assisted synthesis of metastable rare-earth ferrites with widely tunable charge ordering...

Although the d-band correlations within metastable rare-earth ferrites (ReFe₂O₄) enable charge ordering transition functionalities beyond conventional semiconductors, their material synthesis yet requires a reducing atmosphere that is toxic and explosive. Herein, we demonstrate a reactive spark plasma sintering (RSPS) strategy to effectively synthesize metastable ReFe₂O₄ (Re = Er, Tm, Yb, Lu) simply in coarse vacuum within a greatly shortened reaction period. High flexibility is gained in adjusting their rare-earth composition and thereby the charge ordering transition temperature within 218-330 K. Assisted by the temperature-dependent near edge X-ray absorption fine structure (NEXAFS) analysis, an elevation in the Fe³⁺/Fe²⁺ orbital configuration within ReFe₂O₄ was observed compared to previous reports, and it is consistent with their higher Mott temperature and activation energy observed in their electrical transportations. This work elucidates stabilization of the metastable phase (e.g., ReFe₂O₄) via the non-equilibrium processes of RSPS beyond the thermodynamic restrictions.